Linepipe steels are usually microalloyed with Nb to promote the formation of complex microstructures that lead to the required mechanical properties. In particular, Nb in solution affects significantly the austenite decomposition kinetics and the resulting microstructure. A systematic study has been carried out to quantify the influence of Nb on the austenite decomposition kinetics in X80 linepipe steel. Continuous cooling transformation tests were conducted with a Gleeble 3500. The transformation products include ferrite, granular and upper bainite and M/A (martensite/ retained austenite) constituents. For this study optical microscopy was used to investigate the formation of M/A constituents that critically determine the fracture toughness. A relation between M/A and the surrounding microstructure is observed. In combination with an existent model for the prediction of the microstructure evolution during weld thermal cycles, the area fraction, size and morphology of M/A can be predicted for the simulated HAZ, based on the prior austenite grain size, cooling rate and amount of Nb in solution.